Numerous prior art disclosures relate to communications such as the transmission and receiving of data, as well as handshaking that occurs to accomplish the transmission of data from one point to another point where it is received. Many prior art schemes provide data and control signals redundantly to insure correct reception of the data. Among these redundant schemes is a technique referred to as Forward Error Correction (FEC), which is a method used in communication systems that transmits the information multiple times using different encoding methods. FEC is known to improve signal to noise ratio. By using FEC the system has a better chance of transferring the data from the transmitter to the receiver, error free.
In a data communication system as described in U.S. patent application Ser. No. 09/794,693 entitled MULTIPLE-LEVEL PRINTHEAD USING EMBEDDED HIGH SPEED SERIAL DATA AND CONTROL LINK WITH ON-BOARD EXPOSURE CLOCK GENERATION, assigned commonly with the present invention, the bandwidth is almost completely utilized in the transmission of the image data. In such a system, there is not enough bandwidth for Forward Error Correction (FEC) or for the re-transmission of the full line of image data should a data bit or bits become corrupted during transmission. The bandwidth of this prior art system is so limited that, essentially, error-free transmission of digital data must occur in order for the system to operate properly. These control words provide, in part, a means for the receiver to stay synchronized with the transmitter and add only a small increase in the overall bandwidth required. For such a system a corrupted bit used for image reconstruction could be acceptable since this is a very small percentage of the total image data. On the other hand, a corrupted control word could result in the loss of synchronization between the transmitter and receiver. When synchronization is lost, bit alignment at the receiver is lost and the resulting image is corrupted for the duration of the loss of synchronization. In a relatively noise-free environment, this is possibly acceptable since a corrupted transmission of a control character would occur only rarely. In a high noise environment this may prove unacceptable. When this type of system is deployed in a high noise environment, a method is needed such that synchronization between the transmitter and receiver can be more reliably maintained.
In a printing system such as the one described in U.S. patent application Ser. No. 09/794,693, control characters are sent across a communication channel to designate the starting point of a line of image data, the ending point of the line of data, and the time to start an exposure sequence. In the system described in U.S. patent application Ser. No. 09/794,693, the same control character is used to designate the end of the line of data as to designate the start of the line of data. A separate control character is used to designate the start of the exposure sequence. These control characters are sent with each line of image data. The line of image data that is exposed onto the photo-conductor is the line of data that was sent previously. Basically, the imaging element has a one line buffer that holds the ‘just sent’ line of data and then exposes the line of data that was sent on the prior transmission interval.
Electrophotographic printing systems typically employ photoconductive drums that turn at a nominal rate. Electronics on the drum generate signals that are sent from the main machine interface to indicate the time period during which the current line of data should be imaged onto the photoconductive drum. The transmitter accesses these signals and determines the appropriate time to send the next line of data to the receiver. In each case the transmitter sends a start of line character (SOL), the image data, an end of line (EOL) control word, and finally the exposure line (EXP) control word. For the system described in U.S. patent application Ser. No. 09/794,693, the SOL character and the EOL control word have the same bit pattern.
In printing systems as described in U.S. patent application Ser. No. 09/794,693, there are many noise sources. The equipment, in general, requires high voltage power supplies and charging elements. There are many opportunities for arcing to occur, which generates a broadband of spurious noise. In this environment the opportunity exists for these external noise sources to interfere or corrupt the communication channel. Since the transmission of data across the link is tied to the movement of the photoconductive drum, the data must be present at the imaging element when the exposure sequence starts. If erroneous data is stored at the imaging element when the exposure sequence starts, a corrupted image will be produced. Specifically, the integrity and timing of the control characters must be maintained since the loss of a single control character interrupts the synchronization between the transmitter and receiver. The resulting image produced at the photo-conductor will be corrupted until the transmitter and receiver can regain synchronization.
From the foregoing discussion, it is apparent that there remains a need within the art for a method and apparatus that can insure correct data transmission in systems having limited bandwidth.